Wireless communications devices, for instance, mobile phones, have become popular remarkably in recent years with their functions and services improved increasingly. Explanation will be made on a mobile phone as an example. There are various systems for mobile phones, for instance, GSM (global system for mobile communications) and DCS1800 (digital cellular system 1800) systems widely used mostly in Europe, a PCS (personal communications services) system used in the U.S., and a PDC (personal digital cellular) system used in Japan. According to recent rapid expansion of mobile phones, however, a frequency band allocated, to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequency, and further to expand serviceable territories and to effectively use communications infrastructure of each system. Thus, mobile phones adaptable to a plurality of systems are called multiband mobile phones, differentiated from single-band mobile phones adaptable to only a single system.
If a mobile phone is simply provided with circuits for every system to make one mobile phone adapted to a plurality of systems, every system has a transmitting system needing, for instance, a filter for passing a transmission signal of the desired transmission frequency, high-frequency switches for switching a transmission circuit and a reception circuit, and an antenna for receiving and emitting transmitting/receiving signals, and a receiving system needing a high-frequency circuit device such as a filter for passing a reception signal of the desired frequency among those passing the high-frequency switch. Accordingly, the mobile phone is not only expensive, but it also has increased volume and weight, whereby it is not suitable for mobile gears. Thus, to realize a mobile phone capable of handling a plurality of systems, miniaturized high-frequency circuit parts having multiple functions, which can be operated at frequencies for a plurality of systems, are required.
FIG. 25 is a block diagram showing the essential structure of transmission parts in such a multiband mobile phone. This multiband mobile phone comprises an antenna-sharing device for enabling a plurality of systems to be connected to one common antenna, and connected downstream of the antenna-sharing device are a plurality of directional couplers CPn, detecting diodes DKn, error amplifiers AEn, and high-frequency amplifiers PAn, etc. The output end of each directional coupler CPn is connected to the detecting diode DKn to control a transmission output level, such that the electric power of the detected transmission signal is adjusted to a targeted transmission output level. The gain of the high-frequency amplifier PAn is not constant in a frequency band of the system, but may vary depending on the ambient temperature and power supply voltage. Accordingly, every system is provided with a chip-type, directional coupler, so that connecting lines on a circuit board can provide an output proportional to the high-frequency signal to detect the output electric power of the high-frequency amplifier.
As described above, the conventional multiband mobile phones redundantly comprise parts such as directional couplers, detecting diodes, etc. for each system. Because these discrete parts are mounted onto the circuit board, the multiband mobile phones have large transmission parts, resulting in large overall size. In addition, because discrete parts are connected with lines on a circuit board, the characteristics of the transmission parts are deteriorated by the loss of the connecting lines. Further, serious problems with multiband mobile phones that should be miniaturized are increase in mounting areas because of the increased number of parts, the addition of parts such as capacitors, inductors, etc. for impedance matching between the parts, increase in insertion loss, etc.